The embodiments described herein relate generally to permanent magnet electrical machines, and more particularly, to methods and systems for steering magnetic flux using a permanent magnet rotor.
Permanent magnet electrical machines are used in a wide variety of systems operating in a wide variety of industries, such as in pump systems or air handling units. As such, permanent magnet electrical machines are subject to many operating conditions. In such a machine, the torque resulting from the magnetic interaction between the rotor and stator may be reduced by eddy currents generated by magnetic flux flowing through the magnets. Eddy currents are created when a magnet experiences a change in the intensity or direction of the magnetic flux at any point within the magnet. These eddy currents generate resistive losses resulting in less torque production, and therefore a less efficient machine. Furthermore, eddy currents may transform some forms of energy, such as kinetic rotational energy, into heat. Excessive heat produced by the eddy currents also reduces the machine's efficiency and may lead to the failure of an adhesive used to retain the permanent magnets on the spinning rotor. The loss of magnet retention causes significant damages to the machine and may lead to machine failure.
More specifically, rotor magnets passing through the open area between stator teeth, coupled with the attraction to and repulsion from the solid teeth of the stator, cause vibrations, cogging torque, and potentially motor noise, any amount of which may be objectionable to a user. Audible machine noise is unacceptable in many applications. Further, the cogging and the torque pulses at the shaft of the machine may be transmitted to a fan, blower assembly or other driven equipment/end device that is attached to the shaft. In such applications these torque pulses and the effects of cogging may result in operational deficiencies and/or acoustical noise that can be objectionable to an end user of the machine.
Semi-closed stator slots used in known electric machines typically include tooth extensions at the tip of the stator tooth that may concentrate the magnetic flux and minimally dissipate the eddy currents within the permanent magnets. The tooth extensions serve the primary purpose of improving the effective flux distribution between the rotor and the stator and may reduce the eddy currents and cogging torque in a permanent magnet machine. However, semi-closed stator slots typically increase the complexity and expense of coil winding machinery. Stators that incorporate semi-closed slots with tooth extensions are more difficult to fabricate, and the area available for the copper wire windings that can be inserted or wound into such slots is limited. As such, open slot or nearly open slot stators are preferred for manufacturing reasons, as the wire for the windings can be inserted into the slots with greater ease and at reduced cost.